Scrap metal can be recycled into quality steel using an electric arc furnace (EAF). In an EAF, the scrap metal is melted with electric arcs formed to the scrap metal. The scrap metal can include small amounts of non-ferrous metal and the like. The EAF process operates as a batch melting process, producing batches of molten steel. The EAF is a highly effective melting apparatus. A significant fraction of steel produced in the U.S. is produced with an electric arc furnace.
However, a drawback in the EAF manufacture of steel is the production of EAF metallurgical dust waste by-products. EAF dust is generated during the steel making process by a variety of mechanisms, including droplet ejection from the turbulent melt and vaporization. The vaporization mechanism is primarily responsible for the relatively high proportion of the non-ferrous metals in the dust such as zinc, lead, tin, chromium, copper and cadmium. The vaporized metals condense as oxides and ferrites and generally are collected downstream in a baghouse and/or electrostatic precipitator. Due to the presence of non-ferrous metals in the dust, the furnace dust cannot be directly recycled. The production of 1 ton of steel can generate approximately 34 pounds (15.4 kg) of waste EAF metallurgical dust.
The rapid growth of the EAF steel process has made EAF metallurgical dust one of the fastest growing and one of the most significant environmental problems worldwide. At present, there are approximately 600,000 metric tons of EAF waste generated annually in the USA and an additional 600,000 metric tons generated annually in the rest of the world. There are also similar quantities of metallurgical dust at a lower level of contamination that is derived from the other major process for steel manufacturing, the Basic Oxygen Furnace (BOF). Because the levels of toxic metals such as cadmium, lead and zinc are lower in BOF metallurgical dust, BOF dust is not currently classified by the EPA as hazardous. However, BOF metallurgical dust has non-iron contaminants that make it difficult to utilize it in current steel manufacture. Thus, BOF metallurgical dust may end up as unused waste.
EAF metallurgical dust may contain high concentrations of iron, zinc, and lead, and smaller amounts of tin, cadmium, chromium and copper. The remainder of the dust is other components, e.g., silica, lime and alumina. The nonferrous values represent potentially rich sources of metal values. Due to the presence of potentially hazardous metals, such as lead, chromium and cadmium, the EAF dust cannot be disposed in landfills since the hazardous metals may leach out due to rain or underground water to contaminate neighboring water sheds. Thus, the processing of the dust is an important commercial and environmental issue. Some specific examples of metal content for three samples of EAF dust are presented in Table 1.
TABLE 1SAMPLE PLANT EAF DUST CONSTITUENTSFOR THREE DIFFERENT SAMPLES% Zn% Al% Pb% Fe% Cd% Cu% Mn% Na% Ba120.30.271.2736.00.020.163.540.590.01222.70.301.0434.80.010.133.600.700.01327.01.426.00.0813.4% CaO% Cr% Mg% Ni% V% As% SiO2% Cl15.510.202.060.020.01.00362.520.9625.480.202.480.130.02.00294.740.7830.25